48V Inverter/charger vs. MPPT SCC float settings for AGM string

Dusty · July 2018

My XW6048 charger is set to a float voltage of 55.4v for my AGM battery bank (4 cells in series in 2 parallel strings), but the float voltage of my SCC is set to 54.0V by default. Wouldn't that use grid power to keep the batteries topped off instead of PV? I would think the SCC float voltage should be set a little higher than the inverter's charger, but by how much, and what should the voltage ideally be set to? Thanks in advance!

BB. · July 2018

Hopefully, one of the folks that knows a whole lot more than me will come by and give you a better answer.

If you are using an "integrated solution" (Schneider XW inverter + Schneider solar charge controllers) networked together... They should play well together. Favoring the solar charge controller for battery charging--And if you have exporting of power turned on (Grid Tied feedback of power back to the utility)--Then during the day, it is really not an issue (the XW is taking all of the available solar panel energy, not used for charging the battery bank, and feeding it back out to the grid).

If you cannot backfeed to the grid (not all utilities will allow selling to the grid), be aware that many Hybrid inverters will draw upwards of 100-200 Watts from the utility, even if there is more than enough solar power to run the home/charge the battery bank.

It seems to the nature of the beast (different brands/models, same issue of drawing low, but significant, energy from the utility even when solar power is available).

Guessing, but it appears that the Hybrid inverters error on the side of caution to not feed energy back into the grid when it is not allowed.

In practice, your AGM batteries (in good condition, kept at room temperature, not in a "hot room"), will only draw around 0.1% to 0.01% current for float... Say you have a 600 AH @ 48 volt AGM bank, the float power would be around:

54 volts * 600 AH * 0.001 float charge = 32 Watts typical maximum float power

New batteries in a cool location, may be 1/10th that amount of energy... Not a whole lot to worry about (the rest of your system probably consumes more than that to run the XW inverter, solar charge controller, etc.). AGMs just do not have very much self discharge.

When the batteries are near failing (old age, other issues), if float current approaches 2% of battery AH rating, those batteries need to be replaced (and check float voltage, etc.)... At 2% and above, you run the risk of boiling your batteries and even causing a fire/explosion (AGMs should never be that high of float current--Flooded cell lead acid could at end of life).

-Bill

Dusty · July 2018

Thanks Bill. To clarify, I am using the Schneider MPPT 60-150 SCC, and the XW6048 is connected to it by Xanbus. The AGMs are in my attached garage in an enclosure that is currently 78.8 deg F as I write this. I don't have a combox--I have an old version of the XW configuration tool hooked up to my PC.

On the XW6048, the charger settings are as follows; Bulk voltage-57.6, Absorption voltage-57.6, Absorption time-180 min, Float Voltage (greyed out so I can't adjust it)-55.4, battery temp compensation-84.0mv, default battery temp-warm.

In the MPPT60 battery settings; Bulk-57.6, Absorption-57.6, Float-54.0, battery temp comp.-84.0mv.

I have the system set to Enhanced Interactive Mode. (Grid Supp Volts set to 64) I am in Grid Support with Sell mode enabled.

For several years, I've had the 6048's charger turned off to be sure (in my mind) that the MPPT60 was solely responsible for keeping the batteries charged. I also have a Victron BMV-600s monitor, and through all those years, overnight battery SOC has never dipped below 94.8 unless I had a grid outage (maybe 4 times in 6 years and I never went below 60% SOC). As the sun came up, by 0830, the Victron would be back to 100% SOC and I would be selling to grid by then. Throughout the day, the Victron would show only a slight bleeding off of SOC so that by the time the sun went down, SOC would be about 99.3. Overnight, it would bleed down to 94.9 or so, and the cycle would begin again the next day. I have about 3.4 kw (2.6 realized kw) of PV and only 110Ah of 48V battery bank. I know my battery bank is way undersized.

I wasn't sure if leaving the XW6048's charger turned off saved me from accidentally pulling any unwanted power from the grid (if my float voltage settings are wrong) instead of the SCC, since I'm not sure if the float voltage settings between the 6048 charger and the SCC should be manually offset by some amount to ensure the inverter's charger only kicked on if solar couldn't keep up. Since I'm in Grid Support, any power that my PV isn't making to support the loads is automatically pulled from the grid, so I never see any voltage dips in my battery bank as a result.

Also, am I maximizing harvest with a float voltage of 54.0, or will harvest improve with a different float voltage?

Regarding your explanation of "float current," I don't show float current in any of my status displays, so I'm not sure how I would know if it is approaching 2% of the batteries' Ah rating. But that's something to consider--thanks!

mcgivor · July 2018

@Dusty said

Regarding your explanation of "float current," I don't show float current in any of my status displays, so I'm not sure how I would know if it is approaching 2% of the batteries' Ah rating. But that's something to consider--thanks!

The 60-150 MPPT has the 2% built into the firmware, not adjustable, the current is dependent on what battery capacitywas programed into the controller, 100Ah = 2 A, 1000Ah = 20A, and so on, the XW 6048 should be the same. This is the absorption current before transition to float, float current itself would /should be less, the current can be seen on the charge controller LED display.

Dusty · July 2018

mcgivor said:

@Dusty said
Regarding your explanation of "float current," I don't show float current in any of my status displays, so I'm not sure how I would know if it is approaching 2% of the batteries' Ah rating. But that's something to consider--thanks!

The 60-150 MPPT has the 2% built into the firmware, not adjustable, the current is dependent on what battery capacitywas programed into the controller, 100Ah = 2 A, 1000Ah = 20A, and so on, the XW 6048 should be the same. This is the absorption current before transition to float, float current itself would /should be less, the current can be seen on the charge controller LED display.

Thank you! I understood @BB was saying if the float current to my batteries--as they age--approaches 2% of the Ah rating, I should replace them. That would be 2.2A for my small battery bank. I'm not sure if that 2% is for both FLA and AGM batteries. I'm using the XW configuration tool to monitor my SCC charge rate. I'll just have to look more closely at what it's telling me. Right now, as it is getting light outside, the SCC is in Bulk charge, pushing 50.460V @ 1.6A. I'll try to catch it when it leaves absorption and enters float, but by then it will already be supporting loads which I assume will skew the amperage display. I guess to test this, I should disconnect all the loads, disable grid support and then see what the float current is to the batteries from PV?

mcgivor · July 2018

You are correct loads will affect the transition based on the 2%, which is why there is an absorption timer, which overrides the 2% should it never actually reduce to that value, Intermittent loads like a refrigerator generally turn off long enough to initiate the transition, but for the purpose of testing it probably best to disconnect everything, including ac input and watch the controller, for this exercise it would be best to extend the absorption timer to maximum minutes to rely solely on the 2%, if it never reaches this, assuming sufficient PV input, the batteries probably are on the way out, only generating heat within the batteries. The use of a DC clamp on ammeter is very helpful in the diagnosis, should one be available.

BB. · July 2018

It is for any lead acid battery that >~2% float charging current (remember, this is continuous current--Many hours/days/weeks on end). This will cause the battery bank to overheat (basically extended equalizing charge) and worse.

AGM/Sealed/GEL batteries should never even see near this float current as they will fail at lower float currents (AGM typically have a catalyst that will overheat/wear out/vent). GEL will develop gas bubbles in the gelled electrolyte, and all of them will vent (any of these sealed batteries should never vent in normal operation--The venting is a safety device and indicates a permanent loss of water).

-Bil

Dusty · July 2018

I'll be testing it in the next 30 minutes. The absorption cycle is almost finished, so it should be in float soon.

If I disconnect AC1, my sub-panel won't be fed, so that will drop almost everything in the house while I check the float current. Not a big problem, since it shouldn't take very long to do.

Thanks, both of you, for your help!

BB. · July 2018

Just monitor the temperature of the bank when floating--Should not be much warmer than the surroundings (if stable room temperature).

Warm/Hot battery (or battery bank), can indicate issues (unless you are hard charging/EQing--But still watch maximum temperatures are not exceeded).

-Bill

Dusty · July 2018

Will do! Absorption should be done soon. The battery temp sensor reads 77 degrees on the nose, and I felt all the batteries and they are cool to the touch. Not even warm.

The system just went into float. I still have sell mode on, making about 800W.

After turning sell and grid support off, the SCC went back into absorption for a minute, then into float again. I still have AC1 CB closed, and the battery amps is 0.080 (4w). Battery voltage is 54.510. The Midnight Solar E-panel is in bypass, so there are no loads on AC OUT.

After ten minutes in Float, amps to the battery increased to 0.65 and is holding steady there.

So I guess I don't have to disconnect AC1 to see the float current on the batteries?

Estragon · July 2018

As you may already know, current should drop to near zero going from absorb to float (absent loads), and may take some time for voltage to drop to float for a good read on float current. Putting a smallish load on for a bit might speed the process up.

Dusty · July 2018

Estragon said:

As you may already know, current should drop to near zero going from absorb to float (absent loads), and may take some time for voltage to drop to float for a good read on float current. Putting a smallish load on for a bit might speed the process up.

Thanks for the heads up! After 10 minutes in Float, amperage to the battery climbed to 0.65 (with no loads on AC OUT) and is holding steady there.

BB. · July 2018

Getting a DC Current Clamp DMM (Digital Multi-Meter) is probably the easiest way to measure float currents (typically below 0.1% for batteries in good condition, probably near 0.01% for AGM/GEL batteries in good condition--very low self discharge. Forklift/traction batteries tend to have higher float current/self discharge).

When you get below ~1% current, most "system level" monitors are probably going to have difficulties measuring the smaller currents (+/- 1% is probably typical accuracy).

-Bill

Dusty · July 2018

BB. said:

Getting a DC Current Clamp DMM (Digital Multi-Meter) is probably the easiest way to measure float currents (typically below 0.1% for batteries in good condition, probably near 0.01% for AGM/GEL batteries in good condition--very low self discharge. Forklift/traction batteries tend to have higher float current/self discharge).

When you get below ~1% current, most "system level" monitors are probably going to have difficulties measuring the smaller currents (+/- 1% is probably typical accuracy).

-Bill

I see. So the reading I'm getting through the XW configuration tool might not be very accurate, but what it is telling me is that my 6 year-old AGM batteries are taking in float current at about .5% of Ah capacity. I'll have to try the test again with a clamp-on DC ammeter. Thanks again!

BB. · July 2018

I am guessing a bit here... But typically a "good meter" is accurate to something like 1% full scale. With a clamp DMM, you can change the full scale range from (example) 400 Amps to 40 Amps and see the smaller values.

Be a bit careful, the DC current through a clamp meter does have a polarity (+/- sign). You could be charging at 0.5% or discharging at 0.5% if you don't pay attention to the sign.

Also, it is possible you will have some ripple current (single phase AC inverters draw what is known as a Sine Squared current wave form...). That is typical--The battery bank acts like a giant capacitor to "supply average current" to the Inverter (Solar Panels do not have the ability to "surge" current and need capacitors/batteries/etc. to store that tiny bit of energy between AC sine waves).

-Bill

Dusty · July 2018

I broke out my Klein CL2000 clamp-on ammeter, and with no loads and the SCC in Float, I was reading 0.02-0.06 on the meter, while the XW configuration tool said it was 0.08. I waited another 5 minutes and watched the SCC on the XW config tool climb to 0.44, and then the SCC went back into absorption for some reason. It was back in float after about a minute and started out at 0.08A again. I'm not sure why it isn't staying in float, since there are no loads on AC OUT.

I disconnected AC1, and the SCC would still cycle between Float and absorption status. Float would start out at 0.02-0.06 on the ammeter and .08 on the XW config tool, then over the course of 5 minutes, slowly climb to 0.28-0.40 on the config tool and then drop back into absorption. It would only stay in absorption for a minute or two, and the cycle would start over. But taking readings with the clamp-on ammeter over this same period of time, it would start at 0.02-0.06 and never go higher than 0.12 before it dropped back down to 0.02.

BTW, the battery temp sensor never rose above 77 deg F, and each battery was still cool to the touch. I couldn't detect a temp rise in any of them.

BB. · July 2018

See what the battery voltage is doing--It is staying around a "reasonable float voltage"?

As long as the average float current remains low for your bank (less than 1 amp)--You should be doing fine. Don't drive yourself nuts looking too closely under the hood.

The fact that the SCC indicates going into and out of float--That does worry me a bit and I would try to get some information on that (from here and/or from Schneider).

When something does not look quite right--It is usually worth investigating. Things can go "wrong" in the future when you are not at home to check.

-Big

Dusty · July 2018

I never noticed the SCC doing that before, but that's probably because I've been in grid support and sell all of the time.

I'm back in G.S. and Sell, the the SCC has been in absorption since. I don't know if the 180 minute timer starts all over again, so I'll force the SCC to Float and see what happens.

Dusty · July 2018

After forcing the SCC to Float, battery voltage is 52.81V, and I'm making 11.25A 635W (cloudy day). After about 2 minutes, it dropped back to Absorption, but the battery readings didn't change. I'll watch it for the next few hours to see if the absorption timer has to run its course. With all the fiddling I've been doing, I'm not sure if what I'm seeing is a problem or just the result of my fiddling. Thanks again for your help!

It's nice to know that the AGM batteries are still in good shape after 6 years of grid support and selling--even if they've only been used a couple times for backup when my grid went down.

I wondered how the ripple induced in such a small battery bank would impact their longevity. Since I commissioned the system back in 2012, the most I ever peaked was about 1800W, but that's almost twice the output of what my battery bank is recommended to handle, since it's only 110Ah.

BB. · July 2018

As always, test your battery bank with a real load (cut AC power at night, or similar), and see that your battery bank supplies the energy needed for the length of time you expect...

When the battery bank can no longer support your needs--Time for a new bank. 6-7 years of service for good quality AGMs is probably just about what I would expect in a moderate climate (I am no battery expert)... (heat kills batteries).

-Bill

Dusty · July 2018

Will do! I did a test like that last year. I disconnected AC1 and ran my system for a week off grid as part of my hurricane preps. I was able to keep my refrigerator and chest freezer running all night, and the next morning, I was still above 85% SOC. By 10:00am the batteries were fully charged again (according to my Victron BMV-600), and I could start cutting in additional circuits to use the excess PV.

I was doing great until we had a slow-moving Nor'easter come in, and after three days of very rainy weather, my battery bank got down to 60% SOC, so I went back on the grid. But I was happy with how the system performed.

I plan to replace the battery bank with LiFEPO4's because I have limited battery space. With those lithium batteries, I could quadruple my usable storage capacity in the same battery cabinet. And the way I use the batteries as only emergency backup, I may never have to replace them (10k cycles). At least that's my hope! But I won't do that until these AGMs are used up.

BB. · July 2018

It can take time to get the LiFePO4 battery bank, configure, and get the BMS integrated that you want... Don't wait until the last minute as the Nor'easter blows in....

Take care,
-Bill

Dusty · July 2018

My SCC's Recharge Volts setting was 54V, and the manual says, "When the battery
voltage drops below the ReCharge Volts setting for a cumulative

period of 1 minute, a new bulk cycle will be triggered."

Should I set Recharge volts lower? And if so (what do you set yours to?), how much? Digging in to Appendix A, I found that they recommend 50V for Recharge volts.

I remember getting the 54V recommendation on this forum, but since my unit keeps cycling back into Absorption Mode, I'm going to set it to the recommended 50V and see how it runs--unless you have any other recommendation. The inverter's charger was already set to 50V (but I usually leave it disabled).

I think setting it lower will solve the problem of it continually cycling back to absorption.

Thanks!

Estragon · July 2018

Don't recall offhand what mine's set at, buy I'm quite sure it's a lot lower than 54v. Fully charged and resting is ~51v. IIRC, my rebulk is around 48-49v. Much higher than that, if I used the table saw or made toast while a pump was running, that would be a new cycle.

Dusty · July 2018

Estragon said:

Don't recall offhand what mine's set at, buy I'm quite sure it's a lot lower than 54v. Fully charged and resting is ~51v. IIRC, my rebulk is around 48-49v. Much higher than that, if I used the table saw or made toast while a pump was running, that would be a new cycle.

Thanks @Estragon, I think for now I'll leave it set at the default of 50V. Watching my SCP, I'm seeing my battery voltage is 52.5 and I'm only making 550W (cloudy), but it's staying in float now and not always cycling into absorption. It will be interesting to see the beneficial impact on my daily harvest.

Dave Angelini · July 2018

Hard to beat the default settings on the -60. If you were at 54v not to good. 54 is the neighborhood of the sell voltage. Sure that you are not mixing this up?

I did not look at this thread but if no one has said it, you should do a load test. Especially if this is there for emergency.
Get an electric heater or some better load for summer and test down to 50%. You should have consumed 30% of the rating or more. Less than that and you are really rolling the dice with an AGM. They go south much faster than flooded.

Dusty · July 2018

I was given the recommendation for a Recharge voltage setting of 54v, but it was causing my system to continually cycle from float to Absorption. Since the Manual said that the Recharge default is 50V, that's what I changed it to, and the constant cycling from float to absorption stopped. Float voltage is still set to 54V, and GSV is set to 64V to enable Enhanced Interactive Mode.

I've never discharged the batteries down to 50% SOC. 60% SOC is the lowest I ever went, and that was only once or twice. But a load test is a good idea.

Dave Angelini · July 2018

If you don't exercise a battery, you can't expect it work when you need it. Even more so with an AGM where state of charge and even knowing the percentage is a big unknown. I tell my grid clients to do this at least 3 times a year and before hurricane or fire season. It is a good thing to run for 5 days and see if it works

One has to recalibrate the battery monitor with accurate data and very few have ever done this. The Schneider battery monitor has 5 data entry steps and senses the strings and the middle of a string. Even so, I would not trust it on old batteries.

There should be a sticker on the Soc display that comes on after 3 years. Not Calibrated Use for Indication only.

This is why a lithium battery with a designed in BMS and it's own output of Soc is so valuable and accurate.

Dusty · July 2018

I did that last year and ran the system for a week off-grid just to see if I could. I guess I should do that every year, as you suggest. I really do intend to upgrade to a LeFePO4 bank when these AGMs are shot.

48V Inverter/charger vs. MPPT SCC float settings for AGM string

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